1. Field of the Invention
The present invention relates generally to variable-speed motor drives, and more specifically to electronic commutation of a switched reluctance motor. In particular, the present invention relates to indirectly determining rotor position for electronic commutation in order to eliminate the need for a rotor position sensor.
2. Description of the Related Art
Variable speed drives of less than 20 kW preferably use brushless dc drives to obtain high efficiency and flexible control characteristics. In particular, the development of computer aided design tools and efficient power semiconductor devices have made the switched reluctance (SR) motor especially attractive.
The SR motor does not require permanent magnets and produces torque by the variable reluctance principle. A position sensor, however, is required in the SR drive in order to synchronize phase excitation pulses to the rotor position.
The process of deriving correctly phased signals from the rotor shaft position and using them to control the timing of switching operation of the power semiconductor devices in the drive is called "electronic commutation". The speed-torque characteristics of the SR motor can be flexibly controlled by changing the switching angles according to speed and torque requirements.
The sensing of rotor position is usually performed by optical or Hall-effect sensors. This involves mounting the sensors in close proximity to the rotor. Such a position sensor, however, constitutes a substantial fraction of the total system cost and tends to reduce the system reliability. For the appliance industry and particularly for hermetically sealed compressors, the factors of cost and reliability are especially important and have lead to consideration of an alternative technique of determining rotor position.
An indirect method of position sensing is described in McMinn, et al., "Application of the Sensor Integration Techniques to the Switched Reluctance Motor Drive," IEEE Industry Applications Conference Record 1988, pp. 584-588. In this method, short duration, low level voltage pulses are applied to the two unenergized phases of an SR motor and the resulting current pulses are measured to obtain an indication of the impedances of the unenergized motor phases and an estimation of the rotor angle. The circuitry for implementing this method, however, is rather complex and relatively expensive.